Tuesday, January 31, 2012

VHF/UHF Dual Band Antenna "Feature" You Should Know

Most dual band antennas available for VHF/UHF have a little discuss “feature”. 

Maybe you have noticed that very few antenna manufactures ever give their angle of elevation[1] information for each band.  They are very happy to share the maximum gain in dB for each band.  However, there is a characteristic of most antennas that causes the higher frequency signal to leave the antenna at a high angle.  It is not unusual for the major lobe on 2 meters to be less than 10 degrees while the 70 cm major lobe to be higher than 45 degrees.  The truth of the matter is that there will usually be a secondary lobe on 70 cm that is lower than 10 degrees.  But that lobe is not the maximum gain lobe.  Now as far as operational performance, this is seldom a problem.  The only time this may be noticeable is if you find that 2 meter signals are getting difficult to copy and change to 70 cm expecting to get an improvement in signal strength.  You may still find that the signal is easier to copy, but this will most like be due to better signal to noise ratio[2] rather than a stronger signal.

So is this a problem to be concerned with.  Generally speaking I’d say no.  However, you should be aware of it and understand that the gain numbers you see in advertising are necessarily representative of an antennas performance in the real world.

There is one antenna, in my experience, that does have low angle at both VHF and UHF.  It is the ARROW OSJ 146/440.  See http://www.arrowantennas.com/osj/j-pole.html.  It is one of the best antennas for the money.  It is rugged build from aluminum rod not tube.  It has good SWR on both bands.  It is heavier than antennas made of tubing but still not unreasonable.  I have one that I use for portable operation with my RV and just “out in the field”.  It has been blown over while up on a 20 foot mast and didn’t even bend when it landed on a parking lot.  I know a number of ham locally that own one of these and I have not heard one complaint.

[1] - Angle of elevation is the angle above the horizon at which the strongest lobe leaves the antenna.  The smaller the angle number is the better for reaching maximum distance.
[2] - Signal to noise ratio is a measure of signal strength to the background noise, i.e. noise floor.

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